(1) Field of the Invention
The present invention relates to methods of fabricating semiconductor devices, and more specifically to a method used to form a silicon nitride-silicon dioxide composite layer for use as the gate dielectric material.
(2) Description of Prior Art
Reduction in gate dielectric layer thickness has allowed the performance of metal oxide semiconductor field effect transistor (MOSFET), to be increased, while device operating voltages have been reduced. However the continuing down scaling of silicon dioxide gate insulator thickness can present yield and reliability concerns for ultra-thin, less than 20 Angstroms, silicon dioxide layers. The use of a composite dielectric layer such as a silicon nitride-silicon dioxide stack, reduces the yield and reliability concerns encountered with thinner silicon dioxide gate layers, while the higher dielectric constant of silicon nitride component allows a thicker gate insulator stack to be used. In addition the equivalent oxide thickness (EOT), of the silicon nitride-silicon dioxide composite can be comparable to thinner silicon dioxide counterparts, thus preserving the performance objective previously satisfied by thinner silicon dioxide layers, therefore the silicon nitride-silicon dioxide stack, has emerged as a attractive gate replacement for thin silicon dioxide gate layers.
One method of forming the silicon nitride-silicon dioxide stack is to first thermally grow the silicon dioxide component followed by deposition of the overlying silicon nitride layer. However this method can result in unwanted trapped charge at the silicon nitride-silicon dioxide interface, as well as fixed charge generated as a result of the overlying silicon nitride layer, at the silicon dioxide-semiconductor interface. The generation of these charges is the undesired shifts of flatband voltage, and thus erratic threshold voltages.
The present invention will describe methods of forming silicon nitride-silicon dioxide gate stacks wherein the silicon nitride component is first formed followed by formation of the underlying silicon dioxide component. This process sequence results in an improved nitrogen profile in the composite gate stack, as well as a reduction in nitrogen pile up at the silicon dioxide-silicon semiconductor interface, thus resulting in superior device performance when compared to counterparts formed via deposition and anneal of a silicon nitride layer on an already grown silicon dioxide layer. Prior art, such as Tobin et al, in U.S. Pat. No. 5,972,804, Debusk et al, in U.S. Pat. No. 6,140,187, Hu, in U.S. Pat. No. 5,962,904, Hu, in U.S. Pat. No. 6,096,640, and Lee et al, in U.S. Pat. No. 6,204,125 B1, describe methods for forming gate, as well as composite gate insulator layers. None of the above prior arts however describe the novel procedures described in the present invention, in which a composite silicon nitride-silicon dioxide gate insulator stack is formed featuring formation of the underlying silicon dioxide component after deposition of the overlying silicon nitride layer.
It is an object of this invention to form a silicon nitride-silicon dioxide stack for use as a MOSFET gate insulator layer.
It is another object of this invention to initially deposit a silicon nitride layer prior to formation of the underlying silicon dioxide component of the silicon nitride-silicon dioxide, gate insulator stack.
It is still another object of this invention to form the silicon dioxide component of the silicon nitride-silicon dioxide gate insulator stack, via implantation of oxygen ions through the silicon nitride layer into a top portion of the semiconductor substrate, followed by an anneal procedure used to activate the oxygen ions and to form a silicon dioxide layer underlying the silicon nitride layer.
It is still yet another object of this invention to form the silicon dioxide component of the silicon nitride-silicon dioxide gate insulator stack, via ultra-violet generation of oxygen radicals which penetrate through the silicon nitride layer into a top portion of the semiconductor substrate, followed by an anneal procedure used to activate the oxygen radicals, forming a silicon dioxide layer underlying the silicon nitride layer.
In accordance with the present invention methods of forming a silicon nitride-silicon dioxide gate insulator stack, wherein the silicon dioxide component of the gate insulator stack is formed after deposition of the overlying silicon nitride layer, is described. A first embodiment of this invention initiates with the formation of a silicon nitride layer, via chemical vapor deposition (CVD), or via plasma nitridization procedures. An optional anneal cycle employed for densification purposed, can be performed if desired. Implantation of oxygen ions through the silicon nitride layer and into a top portion of the semiconductor substrate, is followed by an anneal procedure used to activate the oxygen ions and to form a silicon dioxide layer located underlying the silicon nitride layer of the silicon nitride-silicon dioxide gate insulator stack.
A second embodiment of this invention again initiates with the formation of a silicon nitride layer, via chemical vapor deposition (CVD), or via plasma nitridization procedures. An optional anneal cycle, employed for densification purposed, can be performed if desired. The silicon nitride layer is then exposed to an ultra violet procedure performed to generate oxygen radicals, and to allow the oxygen radicals to penetrate the silicon nitride layer to react with a top portion of the semiconductor substrate to form a silicon dioxide layer. An anneal procedure is next performed to finalize the oxidation procedure, resulting in a silicon dioxide layer formed using ultra-violet generated oxygen, again located underlying the silicon nitride layer of the silicon nitride-silicon dioxide gate insulator stack.